Why did Blender move from 2.83 straight to 2.9?
Blender foundation wanted to move to a more familiar version numbering system.
Can't find a reliable source to quote at the moment, but I believe I remember reading somewhere that Ton Roosendal established that major Blender versions like 3.0 or 4.0 would have to carry deep API or paradigm changes to warrant a major version bump.
This meant that Blender would be stuck in the 2.0 series for a very long time, unless it was rewritten from the ground up.
After the 2.8 series revamp, newfound success and funding, a switch to a faster and more familiar numbering scheme was adopted, as stated in the Code Blog Post Blender LTS and 3.0

Release numbering Along with this, I also propose to accelerate a bit
our release numbers this decade.
This summer we’ll do Blender 2.90 (new particle nodes), and in summer
2021 the Blender 3.0 series begins! By then we will implement a more
conventional release numbering.
I suggest to do minor releases (3.0, 3.1, 3.2, … 3.7) for two-year
periods, and then move to a new major release. Blender 4.0 could be
there in 2023 already!
Below is a summary. Let us know in the comments below what you think

This way introduction of major features like the waited Particle Nodes warrants a major bump.